It is desirable to apply mineralized and/or ceramic coatings to a variety of articles. Biological implants (e.g., medical implants) represent one class of articles to which such coatings are frequently applied. The substrate to which such a coating is applied is usually a metal or a plastic, but the coating can be applied to other substrates such as ceramic and silicon.
Biological implants, such as joint and dental prostheses, usually must be permanently affixed or anchored within bone. In some instances it is acceptable to use a bone cement to affix the prosthesis within bone. In the case of many joint prostheses, however, it is now more common to affix the joint prosthesis by encouraging natural bone ingrowth in and around the prosthesis. Bone-to-implant interfaces that result from natural bone ingrowth tend to be stronger over time and more permanent than are bone cement-prosthesis bonds.
Optimal bone ingrowth requires that natural bone grow into and around the prosthesis to be implanted. Bone ingrowth and prosthesis fixation can be enhanced by providing irregular beaded or porous surfaces on the implant. Although various materials, including titanium alloys, are biocompatible, they are not necessarily bioactive because they can neither conduct bone formation nor form chemical bonds with bone.
Thus, enhanced fixation of implants within bone can be attained by coating the implant with a bioactive mineralized and/or ceramic material. Such coatings have been shown to encourage more rapid bone ingrowth in and around the prosthesis.
Various techniques are used to apply mineralized and/or ceramic coatings to bioimplantable substrates. These coatings are typically made of ceramics and tend to be characterized by a relatively large crystal size. These coatings can be applied by a variety of techniques including plasma spraying, ion implantation, and sol-gel processing. These coating methods, although relatively widely used, do have some drawbacks. For example, the applied coatings tend to possess micropores and macropores, and they can be relatively thick and brittle. These coatings can also possess chemical defects, and they do not always adhere well to substrates. Finally, such coatings are not evenly and uniformly applied to surfaces with complex geometries, such as porous surfaces with undercut regions. Moreover, surfaces having such complex geometries sometimes are not completely coated.
It has been well documented that calcium phosphate ceramics, especially hydroxyapatite, can conduct bone formation. Hydroxyapatite ceramic has been successfully applied as a coating on cementless metallic implants to achieve quick and strong fixation. Thermal plasma spraying is one of the more common methods used to produce hydroxyapatite coatings. However, the resulting plasma-sprayed hydroxyapatite coating is of relatively low density and is not uniform in structure or composition. The adhesion between the coating and substrate is generally not very strong, especially after long-term exposure within the body. The generation of hard ceramic particles, resulting from the degradation of thermal plasma sprayed coating, and coating delamination, are major concerns.
Low temperature processes have also been implemented to produce apatite ceramic coatings using water-based solutions. Since aqueous solutions can reach any open space, these low-temperature processes can be efficiently used in the case of substrates with complex surface geometries. The hydroxyapatite coating that is formed from this solution can be more biologically friendly to bone tissue than is the plasma-sprayed hydroxyapatite coating which is produced by a high temperature process. However, currently known low temperature processes typically require pretreatment of the substrate.
One example of an aqueous system-based coating technique is disclosed in U.S. Pat. No. 5,205,921 in which bioactive ceramic coatings are electrodeposited upon a substrate. Bunker et al., Science, 264: 48-55 (1994), disclose a technique for applying an octacalcium phosphate upon a substrate by immersing the substrate in a solution containing calcium chloride after surface treating the substrate with a material such as chlorosilane. Other techniques, such as disclosed in Japanese Patent Application No. 8-40711, form a hydroxyapatite coating by exposing the substrate to calcium phosphate in a pressure reactor. U.S. Pat. No. 5,188,670 discloses a technique for forming a hydroxyapatite coating on a substrate by directing a stream of liquid containing hydroxyapatite particles to apply a fibrous, crystalline coating of hydroxyapatite to the substrate.
The bioactivity and stability of synthetic apatite ceramics, such as the calcium phosphate hydroxyapatites described above, have been improved upon by adding elements such as silicon, magnesium, fluorine, and strontium ions to the apatite to substitute for calcium. For example, U.S. Pat. No. 6,312,468 discloses a silicon-substituted apatite that is more bioactive than calcium phosphate hydroxyapatite, and can be used as a synthetic bone material. Also, U.S. Pat. No. 6,338,810 discloses a biocompatible strontium-substituted apatite ceramic produced by mixing calcium phosphate and strontium phosphate powders that can be used, for example, in bone prosthetics. A bioactive bone cement comprising a strontium-containing hydroxyapatite is disclosed in International Patent Application No. WO 01/49327. U.S. Patent Application Publication No. 2002/0127711 A1 discloses a calcium hydroxyapatite matrix used as a support for implanting bone cells grown ex vivo, in which calcium can be replaced by other ions such as barium, strontium, and lead. U.S. Pat. No. 5,441,536 discloses a method for producing an implant involving coating the implant with calcium phosphate that is not apatite, and using hydrothermal treatment to convert the calcium phosphate into an apatite ceramic layer. The calcium ions in the calcium phosphate layer can be substituted with strontium, magnesium, chlorine, fluorine, or carbonate ions during the transformation of the non-apatitic calcium phosphate to apatite.
Despite the existence of numerous ceramic coatings and the various processes for producing such coatings, there remains a need for additional methods of making implantable articles that desirably have improved bioactive ceramic coatings. The invention provides such a method for making implantable articles. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.